Has our war on microbes left our immune systems prone to dysfunction?

An Epidemic of Absence: A New Way of Understanding Allergies and Autoimmune Diseases could be co-marketed with the Thomas Rockwell’s children’s classic How to Eat Fried Worms. It begins with the author, Moises Velasquez-Manoff, recounting his border-crossing to Tijuana to infect himself with Necator americanus—hookworms—in an attempt to cure the asthma, hay fever, food allergies, and alopecia that had plagued him since childhood. In the next three hundred pages, the author very cogently explains the idea that led him to willingly infect himself with a parasite known to cause severe diarrhea, anemia, and mental retardation in children.

Velasquez-Manoff marshals the reams of evidence researchers have accumulated to support said concept: the hygiene hypothesis, but with an updated, parasitic twist. The ideas he presents haven’t been accepted by many in the medical community, and there’s little high-quality evidence, in the form of well controlled trials, that exposure to parasites could have positive effects on human health. So, even if the author is thorough, it’s important to keep in mind that the evidence he’s presenting is primarily in the form of correlations.

The Hygiene Hypothesis

A simplistic view of the hygiene hypothesis is that in the absence of something dangerous to fight against—the cholera toxin, for example—immune cells get confused, or bored, and fight against harmless stimuli like dust mites and peanuts instead. But there is a more nuanced view. Our immune systems co-evolved with an enormous community of microbes, and were in fact shaped by them. Many became established, long-term, and vital residents in our guts; the importance, and in fact the very existence, of these commensals has only recently been realized.

Constant exposure to all of these bugs, as a unit, enhanced the regulatory arm of the immune system, modulating responses so that we could tolerate the filthy environment in which we lived while at the same time (hopefully) fighting off those pathogens that posed a mortal threat and not destroying our own bodies in that process. In the martial analogy that is inevitable in discussing immunology, ancient human immune cells that were always surrounded by microbes were like battle-hardened old soldiers who have learned the ability to watch warily when encountering something new, waiting to see whether or not it is dangerous; modern immune cells raised in our hyper-sanitized environment are like new recruits just given their first gun, testy and jumpy at the first hint of a threat and liable to blow up their surroundings in inappropriately directed and outsized force. Experience has not taught them moderation.

Seeing worms everywhere

Yes, he includes autism in the list of modern diseases caused by our out-of-whack immune systems. Along with other cases where immune dysfunction hasn’t been established, like obesity, cardiovascular disease, type 2 diabetes, and cancer.

There are some serious problems with blaming all of these on immune dysfunction, but we’ll focus on a single example: autism. Just as the absence of worms’ mediating effects on our immune system causes some people to have an allergic response to harmless ingested proteins and others to attack their own tissues, the argument goes, chronic inflammation in the womb generates fetuses with autism.

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